This invention relates to a vent-forming apparatus and methods of using it in metal casting applications.
To make high quality metal castings, one conventional method is the ceramic shell casting process for lost wax casting of ferrous and nonferrous alloys such as steel, aluminum and bronze. According to this casting process, a ceramic shell-type mold is first constructed.
To construct such a shell-type mold, a pattern of the object to be cast in metal is first made from wax, plastic or other soluble, fusible or combustible material. This pattern of the object to be cast in metal is attached to a pattern of a system of ingates, down sprues or runners, and pouring basins. The pattern of the system of ingates, down sprues or runners, and pouring basins is made from wax, plastic or other soluble, fusible or combustible material. Pouring basins, down sprues or runners, and ingates ultimately are used to introduce molten metal into the mold cavity.
The pattern of the object to be cast and the pattern of the system of ingates, down sprues or runners, and pouring basins are coated with multiple layers of ceramic slurry and one or more silica refractory powders or other refractory powers. Silica and other refractory powders are commonly referred to as stuccos. Application of upwards of 30 or more layers is known in the art.
Once the ceramic slurry and stucco layers have dried adequately, a dewax and burn-out cycle is conducted. During the dewax and burn-out cycle, all of the patterns are dissolved, melted or burned away.
Completion of the dewax and burn-out cycle results in a shell-type mold having a hollow space in the form of the object to be cast in metal. This hollow space is commonly referred to as the mold cavity. Completion of the dewax and burn-out cycle also produces the cavities called ingates, down sprues or runners, and pouring basins in the shell-type mold. The mold cavity connects to the ingates, and down sprues or runners which in turn are connected to the pouring basins. Molten metal is introduced into the mold cavity via the pouring basins and the down sprues, runners and ingates.
Molten metal is introduced into the mold cavity in order to cast the desired object. Once the molten metal has solidified sufficiently, the mold is broken or torn away from the cast metal object and discarded. The molds typically cannot be reused.
Shell-type molds used in the ceramic shell casting process typically are refractory molds having only slight gas permeability. These shell-type molds suffer a functional disadvantage resulting from the fact that the mold walls have only slight gas permeability. This characteristically low gas permeability often prevents the mold cavity from adequately filling out with molten metal in heavily detailed sections and in sections having large surface areas relative to volume (i.e., thin-walled sections). This is because during the casting process, gasses trapped in these sections of the mold cavity cannot pass through the low permeability mold walls before the molten metal solidifies. As a result, the finished cast metal object exhibits non-fill defects.
Attempts at solving this fill-out problem include extension of the molten life of the metal by increasing the temperature of the shell-type mold and by increasing the pouring temperature of the molten metal. This approach is not entirely satisfactory as it can result in finished cast objects having coarse, porous structures and gross cracking.
Other attempts at solving this fill-out problem have involved mechanisms for increasing the gas permeability of the shell-type mold walls. Such increases in gas permeability have been achieved by the creation of voids and pores in the shell-type mold walls. These voids and pores in the shell-type mold walls act as conduits to pass gasses out of the mold cavity through the mold walls. However, increasing gas permeability in this manner has drawbacks. If the voids or pores become too large, molten metal will penetrate the mold walls and cause the finished cast object to have an undesirably rough surface. Faced with the possibility of such surface roughness, metal casters often choose to forego this approach.
Instead, some metal casters incorporate venting systems into the shell-type molds. These venting systems typically involve arrays of interconnected vent channels that pass through the shell-type mold walls. These vent channel arrays are located so as to connect hard-to-fill areas of the mold cavity to the atmosphere.
The vent channel arrays are formed in the walls of the shell-type mold as part of the mold making process. Typically, patterns of vent channel arrays are fashioned from wax, plastic or other soluble, fusible or combustible material. Normally, a similar material is used to make the pattern of the object to be cast. These vent channel array patterns are attached, in desired areas, to the pattern of the object to be cast. The pattern of the object to be cast, including the attached vent channel array patterns, and the patterns of the ingates, pouring basins and down sprues or runners are coated with layers of ceramic slurry and stucco to make the shell-type mold as described above.
Openings are cut in desired areas of the mold walls to expose the vent channel array patterns to the atmosphere prior to the dewax and burn-out cycle described above. During the dewax and burn-out cycle, all of the patterns are dissolved, melted or burned away. This results in a shell-type mold having a mold cavity, ingates, pouring basins, down sprues or runners and having walls that are infiltrated by arrays of interconnected vent channels connecting the mold cavity to the atmosphere. As an alternative, the openings to the atmosphere also may be made following the dewax and burn-out cycle, in which case the vent channel arrays themselves are exposed to the atmosphere after the dewax and burn-out cycle. However, it is more desirable to make the openings to the atmosphere prior to the dewax and burn-out cycle because the dissolved, melted or burned materials can run out of the openings to the atmosphere.
The vent channel arrays also may connect to the patterns for the ingates, pouring basins, and down sprues or runners in order to create vents to the atmosphere. In such case, the vents created exhaust gasses to the atmosphere through the ingates, down sprues, runners and pouring basin.
This known venting method is less than optimal because it involves construction, as part of the mold making process, of patterns of awkward and fragile interconnected vent channel arrays made of wax, plastic or other soluble, fusible or combustible materials. Such vent channel array patterns are often damaged or broken during the mold making process when the pattern of the object to be cast in metal, including the attached vent channel array patterns, is dipped into viscous ceramic slurries or turbulent fluid beds. One remedy for this fragility is to increase the diameters of the channels in the vent channel array patterns--channel diameters of upwards of 0.25 inch are not uncommon. However, this solution increases costs in terms of materials and in terms of labor associated with the retooling required for removal of artifacts caused by the large diameter vent channels on the surface of the finished casting.
Furthermore, even if the vent channel array patterns escape damage in the mold making process, successful venting during metal casting is not guaranteed. This is because the path by which molten metal fills the mold cavity is very unpredictable, and molten metal may enter and block a vent channel before gasses in sections of the mold have been adequately exhausted. In such a case, the fill-out problem will not have been solved.
Other problems associated with this traditional venting technique are the result of the fact that the vent channels in the mold walls ultimately are open to the atmosphere. For instance, a vent channel may not shut-off once gasses in the area being vented by that vent channel are evacuated. This shut-off failure can cause indentations and other defects on the surface of the cast metal object or even loss of the casting. In addition, dirt or other foreign matter may enter the mold cavity through the vent channels, fouling the casting.
It would be desirable to provide a venting mechanism which ensures that mold cavities sufficiently fill out with molten metal in heavily detailed sections and in sections having large surface areas relative to volume, thereby providing greater detail and more faithful reproduction in the finished cast object.
It would also be desirable to provide a venting mechanism which does not rely on use of patterns of interconnected vent channel arrays, thus minimizing pattern fragility and simplifying the mold making process.
It would further be desirable to provide a venting mechanism which provides adequate ventilation while minimizing the amount of retooling of the surface of the cast metal object that is necessary.
It would still further be desirable to provide a venting mechanism which minimizes the possibility of molten metal entering a vent before the gasses being exhausted by that vent have been adequately removed.
It would yet further be desirable to provide a venting mechanism which provides adequate ventilation while minimizing drainage of molten metal from the mold cavity.
It would yet further be desirable to provide a venting mechanism which provides adequate ventilation while minimizing the chances for introduction of dirt or other foreign matter into the mold cavity.